Electron gun for cathode ray tube having third to fifth electrodes with different sized electron beam through holes

ABSTRACT

An electron gun for a cathode ray tube is disclosed, which has a simple structure and can prevent the spot on a screen from being degraded. In the electron gun for a cathode ray tube including a cathode that emits electron beams, a first electrode that controls the electron beams emitted from the cathode, a second electrode that accelerates the electron beams emitted from the first electrode, and third to fifth electrodes sequentially arranged in a screen direction to act as pre-focus lenses, the electron gun is characterized in that the third to fifth electrodes have different sized electron beam through holes.

This application claims the benefit of the Korean Application No.P2001-44873 filed on Jul. 25, 2001 and the Korean Application No.P2002-23428 filed on Apr. 29, 2002, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cathode ray tube, and moreparticularly, to an electron gun for a cathode ray tube that can reducea change in the size of a spot due to a change of a focus voltage and achange of current.

2. Discussion of the Related Art

Generally, a cathode ray tube includes an in-line electron gun thatemits three electron beams, a deflection yoke that deflects the electronbeams in a predetermined place of a screen, a shadow mask that selectsthe electron beams, and a screen that reproduces a picture image bycolliding with the electron beams.

According to the Japanese Patent Laid-Open No. 60-51775, a typicalelectron beam spot becomes great if beam current increases. Therefore,to obtain a fine picture image, the beam current should be within thesmaller range if possible. However, since a cathode ray tube thatrequires high current has a great change of current, a uni-bi potentiallens structure having an improved pre-focus area in a bi potential mainlens structure has been employed to reduce the spot size on the screen.

A related art electron gun for a cathode ray tube will be described withreference to FIG. 1.

Referring to FIG. 1, the related art electron gun includes a cathode Kthat emits three electron beams of R, G, and B, a first electrode 1 thatcontrols the electron beams emitted from the cathode K, a secondelectrode 2 that accelerates a thermal electron emitted from the firstelectrode 1, third, fourth and fifth electrodes 3, 4, and 5 that focusthe electron beams, and a sixth electrode 6 that acts as an anode.

The operation of the aforementioned related art electron gun will now bedescribed.

If a heater provided inside the cathode K heats the cathode K, theelectron beams are emitted. The emitted electron beams are controlled bythe first electrode 1 that acts as a control electrode. Also, theemitted electron beams are accelerated by the second electrode 2 andfocused by the third to sixth electrodes 3, 4, 5, and 6.

Meanwhile, if high current is generated from the electron gun, thecurrent density of crossover does not increase by an increased value ofthe beam current due to the space charge repulsion. The current densityis uniformly distributed without forming a Gaussian distribution,thereby degrading the crossover. If the crossover is degraded, the spoton the screen is degraded accordingly.

To prevent the crossover from being degraded, it is necessary to reducethe potential of the crossover, thereby reducing the space chargerepulsion. To increase a voltage of the crossover, the third electrode 3moves to the second electrode 2. Thus, the potential of the crossoverincreases while the space charge repulsion decreases.

However, in this case, an emitting angle α increases and thus the sizeDb of the electron beam in a main lens increases. As shown in FIGS. 7and 8, if the size Db of the electron beam in the main lens increases,spherical aberration increases. In this case, a problem arises in thatthe size of the spot on the screen increases.

To solve such a problem, it is necessary to reduce the emitting angleafter the crossover passes. Since the crossover moves over the secondelectrode 2 under the high current, it is difficult to reduce theemitting angle by means of the third to fifth electrodes 3, 4, and 5.

To reduce the emitting angle α after the crossover passes, anotherpre-focus lens may be provided between the pre-focus lens by the secondand third electrodes 2 and 3 and the main lens.

As shown in FIG. 1, the pre-focus lens is formed in a uni-potential lensstructure by dividing a focus electrode into the third, fourth, andfifth electrodes 3, 4, and 5 and applying the same voltage to the thirdand fifth electrodes 3 and 5. At this time, an electron beam throughhole 41 of the fourth electrode 4 has the same size as that of anelectron beam through hole 51 of the fifth electrode 5. The electronbeam through hole 51 of the fifth electrode 5 is formed in a directionof the fourth electrode. The electron beam through hole 41 of the fourthelectrode is greater than an electron beam through hole 31 of the thirdelectrode 3.

In this case, the emitting angle of the electron beams entered into themain lens decreases. This decreases the size Db of the electron beam inthe main lens. If the size Db of the electron beam in the main lensdecreases, the spherical aberration decreases. As a result, the size ofthe spot on the screen decreases.

However, the aforementioned related art electron gun has severalproblems.

As described above, in the related art electron gun, the fourthelectrode is formed in a plate shape in the pre-focus lens at the frontof the main lens so that the emitting angle of the electron beams isadjusted. At this time, the fourth electrode is adjacent to the thirdelectrode while the fifth electrode is adjacent to the fourth electrode.In forming the pre-focus lens, the electron beam through hole of thefourth electrode has the same size as that of the fifth electrode. Inthis case, design factors that can adjust the emitting angle are limitedto each thickness of the third, fourth, and fifth electrodes, thedistance d1 between the third electrode and the fourth electrode, thedistance d2 between the fourth electrode and the fifth electrode, thesize of the electron beam through hole of the third electrode, the sizeof the electron beam through hole of the fourth electrode, and the sizeof the electron beam through hole of the fifth electrode. As a result,to additionally adjust the emitting angle, supplementary electrodes arerequired among the second electrode, the third electrode, and the fourthelectrode. This causes a complicated structure.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an electron gun for acathode ray tube that substantially obviates one or more problems due tolimitations and disadvantages of the related art.

An object of the present invention is to provide an electron gun for acathode ray tube that has a simple structure and can prevent the spot ona screen from being degraded.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, inan electron gun for a cathode ray tube including a cathode that emitselectron beams, a first electrode that controls the electron beamsemitted from the cathode, a second electrode that accelerates theelectron beams emitted from the first electrode, and third to fifthelectrodes sequentially arranged in a screen direction to act aspre-focus lenses, the electron gun is characterized in that the third tofifth electrodes have different sized electron beam through holes.

Preferably, each electron beam through hole of the third and fourthelectrodes is smaller than the electron beam through hole of the fifthelectrode.

Preferably, the electron beam through hole of the third electrode issmaller than that of the fourth electrode.

In another aspect of the present invention, the electron beam throughhole of the fourth electrode has a rectangular shape and its verticallength is different from its horizontal length.

In another aspect of the present invention, the third electrode has afirst through hole opposite to the second electrode and a second throughhole opposite to the fourth electrode.

Preferably, the size of the first through hole is different from that ofthe second through hole. More preferably, the size of the first throughhole is smaller than that of the second through hole.

Therefore, in the present invention, it is possible to effectivelyprevent the spot from being degraded even if no separate supplementaryelectrode is provided.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a schematic view illustrating a related art electron gun for acathode ray tube;

FIG. 2 is a schematic view illustrating an electron gun for a cathoderay tube according to the first embodiment of the present invention;

FIG. 3 is a conceptional view illustrating the potential distribution ofan electron gun for a cathode ray tube according to the presentinvention;

FIG. 4 illustrates an optical model of an electron lens of an electrongun for a cathode ray tube according to the present invention;

FIG. 5 is a schematic view illustrating an electron gun for a cathoderay tube according to the second embodiment of the present invention;

FIG. 6 is a schematic view illustrating an electron gun for a cathoderay tube according to the third embodiment of the present invention;

FIG. 7 a is a graph illustrating the current density distribution on ascreen in case of no spherical aberration;

FIG. 7 b is a graph illustrating the current density distribution on ascreen in case of spherical aberration;

FIG. 8 a illustrates a shape of a spot on a screen in case of nospherical aberration; and

FIG. 8 b illustrates a shape of a spot on a screen in case of sphericalaberration.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

An electron gun for a cathode ray tube according to the first embodimentof the present invention will be described with reference to FIG. 2.

Similarly to the related art electron gun, an electron gun for a cathoderay tube according to the first embodiment of the present inventionincludes a cathode K that emits electron beams, a first electrode 11that controls the electron beams emitted from the cathode K, a secondelectrode 12 that accelerates the electron beams, a pre-focus lens ofthird, fourth and fifth electrodes 13, 14 and 15 that control anemitting angle of the electron beams, and fifth and sixth electrodes 15and 16 that constitute a main lens part.

In the first embodiment according to the present invention, the thirdelectrode 13 is arranged to oppose and be adjacent to the fourthelectrode 14. The fourth electrode 14 is arranged to oppose and beadjacent to the fifth electrode 15. The third electrode 13, the fourthelectrode 14, and the fifth electrode 15 have different electron beamthrough holes 311, 411 and 511, respectively.

For example, the electron beam through hole 411 of the fourth electrode14 may be greater than the electron beam through hole 311 of the thirdelectrode 13. The electron beam through hole 511 of the fifth electrode15 may be greater than the electron beam through hole 411 of the fourthelectrode 14.

Alternatively, the electron beam through hole 311 of the third electrode13 and the electron beam through hole 411 of the fourth electrode 14 maybe smaller than the electron beam through hole 511 of the fifthelectrode 15.

In this case, an emitting angle α of the electron beam and the size ofthe electron beam in the main lens part may easily be varied.

The principle of the present invention will be described in more detailwith reference to FIGS. 3 and 4.

Referring to FIGS. 3 and 4, a lens L1 denotes an emitting lens by thethird and fourth electrodes, a lens L2 denotes a focus lens by thethird, fourth, and fifth electrodes, and a lens L3 denotes an emittinglens by the fourth and fifth electrodes.

In FIG. 3, the electron beam through hole 411 of the fourth electrode 14is greater than the electron beam through hole 311 of the thirdelectrode 13. The electron beam through hole 511 of the fifth electrode15 is greater than the electron beam through hole 411 of the fourthelectrode 14. In this case, the intensity of the lens L2 becomes morerobust than that of the related art electron gun (the electron beamthrough hole 311<the electron beam through hole 411=the electron beamthrough hole 511). Thus, the emitting angle α of the electron beam tothe main lens and the size Db of the electron beam in the main lens candecrease. A decrease of the emitting angle α of the electron beam andthe size Db of the electron beam reduces the spherical aberration,thereby reducing the size of a spot on a screen.

If the emitting angle α of the electron beam and the size Db of theelectron beam in the main lens depart from an optimal value, the size ofthe electron beam through hole 411 of the fourth electrode 14 isadjusted appropriately. That is, if the electron beam through hole 411of the fourth electrode 14 becomes great, the intensity of the lens L2is weaker than the intensity of the lenses L1 and L3. Thus, the emittingangle α of the electron beam and the size Db of the electron beam in themain lens become great. On the other hand, if the electron beam throughhole 411 of the fourth electrode 14 becomes small, the intensity of thelens L2 becomes more robust than the intensity of the lenses L1 and L3.Thus, the emitting angle α of the electron beam and the size Db of theelectron beam in the main lens become small.

Meanwhile, the electron beam through holes 311, 411, and 511 are notlimited to shapes suggested in the present invention. That is, theelectron beam through holes may have a circular shape, a rectangularshape, or the like. As shown in FIG. 6, if the electron beam throughhole 411 of the fourth electrode has a rectangular shape, it ispreferable that its vertical length 411 h is different from itshorizontal length 411 w. This is because the emitting angle α of theelectron beam in vertical and horizontal directions and the size Db ofthe electron beam in the main lens can be adjusted.

An electron gun for a cathode ray tube according to the secondembodiment of the present invention will be described with reference toFIG. 5.

In the above embodiment, while the third and fourth electrodes 13 and 14have plate shapes, they are not limited to plate shapes. That is, thethird electrode 13 and/or the fourth electrode 14 may have a cylindricalshape. The third electrode 13 may have a first through hole 311 aopposite to the second electrode 12 and a second through hole 311 bopposite to the fourth electrode 14. In this case, it is preferable thatthe size of the first through hole 311 a is different from the size ofthe second through hole 311 b. More preferably, the size of the firstthrough hole 311 a is smaller than the size of the second through hole311 b.

The fourth electrode 14 may also have a first through hole 411 aopposite to the third electrode 13 and a second through hole 411 bopposite to the fifth electrode 15.

Meanwhile, it is preferable to satisfy the relation of {the size of theelectron beam through hole 511 of the fifth electrode×0.1}≦the size ofthe electron beam through hole 311 of the third electrode 13≦{the sizeof the electron beam through hole 511 of the fifth electrode×0.5}. Thisis because the assembly of the electron gun is not easy if the size ofthe electron beam through hole 311 of the third electrode is smallerthan {the size of the electron beam through hole 511 of the fifthelectrode×0.1} while aberration of a tripod increases to increase thesize of the spot on the screen if the size of the electron beam throughhole 311 of the third electrode is greater than {the size of theelectron beam through hole 511 of the fifth electrode×0.5}.

Furthermore, it is preferable to satisfy the relation of {the size ofthe electron beam through hole 511 of the fifth electrode×0.5}≦the sizeof the electron beam through hole 411 of the fourth electrode 14≦{thesize of the electron beam through hole 511 of the fifth electrode}. Thisis because that the emitting angle decreases considerably to depart froman optimal emitting angle, thereby increasing the size of the spot onthe screen if the size of the electron beam through hole 411 of thefourth electrode is smaller than {the size of the electron beam throughhole 511 of the fifth electrode×0.5} while assembly of the electron gunis not easy if the size of the electron beam through hole 411 of thefourth electrode is greater than {the size of the electron beam throughhole 511 of the fifth electrode}.

As aforementioned, the electron gun for a cathode ray tube according tothe present invention has the following advantages.

It is easy to design the emitting angle of the electron beam to the mainlens and the size of the electron beam in the main lens. That is, theemitting angle and the size of the electron beam can be reduced byadjusting the respective size of the electron beam through holes of thethird to fifth electrodes. This can reduce the spherical aberration andcan prevent the spot on the screen from being degraded.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. An electron gun for a cathode ray tube comprising a cathode thatemits electron beams, a first electrode that controls the electron beamsemitted from the cathode, a second electrode that accelerates theelectron beams emitted from the first electrode, and third to fifthelectrodes sequentially arranged in a screen direction to act aspre-focus lenses, wherein the third to fifth electrodes have electronbeam through holes, all of which are different in size from one another,and wherein each electron beam through hole of the third and fourthelectrodes is smaller than the electron beam through hole of the fifthelectrode, and the electron beam through hole of the third electrode issmaller than that of the fourth electrode, and the size of the electronbeam through hole of the third electrode is greater than the size of theelectron beam through hole of the fifth electrode×0.1 and less than thesize of the electron beam through hole of the fifth electrode×0.5. 2.The electron gun for a cathode ray tube as claimed in claim 1, whereinthe electron beam through hole of the fourth electrode has a rectangularshape and its vertical length is different from its horizontal length.3. The electron gun for a cathode ray tube as claimed in claim 1,wherein the third electrode has a first through hole opposite to thesecond electrode and a second through hole opposite to the fourthelectrode.
 4. The electron gun for a cathode ray tube as claimed inclaim 3, wherein the size of the first through hole is different fromthat of the second through hole.
 5. The electron gun for a cathode raytube as claimed in claim 4, wherein the size of the first through holeis smaller than that of the second through hole.
 6. The electron gun fora cathode ray tube as claimed in claim 1, wherein the size of theelectron beam through hole of the fourth electrode is greater than thesize of the electron beam through hole of the fifth electrode×0.5 andless than the size of the electron beam through hole of the fifthelectrode.
 7. The electron gun for a cathode ray tube as claimed inclaim 1, wherein an electron beam spot does not degrade.
 8. The electrongun for a cathode ray tube as claimed in claim 1, wherein the thirdelectrode has a first through hole opposite to the second electrode anda second through hole opposite to the fourth electrode, and the fourthelectrode has a first through hole opposite to the third electrode and asecond through hole opposite to the fifth electrode.
 9. An electrode gunfor a cathode ray tube comprising a cathode that emits electron beams, afirst electrode that controls the electron beams emitted from thecathode, a second electrode that accelerates the electron beams emittedfrom the first electrode, wherein third to fifth electrodes sequentiallyarranged in a screen direction to act as pre-focus lenses, and the thirdto fifth electrodes have electron beam through holes, all of which aredifferent in size from one another, and the size of the electron beamthrough hole of the third electrode is greater than the size of theelectron beam through hole of the fifth electrode×0.1 and less than thesize of the electron beam through hole of the fifth electrode×0.5, andwherein no separate supplemental electrode is provided.
 10. An electrongun for a cathode ray tube comprising: a cathode emitting electronbeams; a first electrode controlling the electron beams emitted from thecathode; a second electrode accelerating the electron beams emitted fromthe first electrode; and third to fifth electrodes sequentially arrangedin a screen direction to act as pre-focus lenses, the third to fifthelectrodes having electron beam through holes, all of which aredifferent in size from one another, and the size of the electron beamthrough hole of the third electrode is greater than the size of theelectron beam through hole of the fifth electrode×0.1 and less than thesize of the electron beam through hole of the fifth electrode×0.5. 11.The electron gun as claimed in claim 10, wherein each electron beamthrough hole of the third and fourth electrodes is smaller than theelectron beam through hole of the fifth electrode.
 12. The electron gunas claimed in claim 11, wherein the electron beam through hole of thethird electrode is smaller than that of the fourth electrode.
 13. Theelectron gun as claimed in claim 10, wherein the size of the electronbeam through hole of the fourth electrode is greater than the size ofthe electron beam through hole of the fifth electrode×0.5 and less thanthe size of the electron beam through hole of the fifth electrode. 14.The electron gun as claimed in claim 10, wherein the beam through holeof the fourth electrode is smaller than the electron beam through holeof the fifth electrode adjacent to the beam through hole of the fourthelectrode.
 15. The electron gun as claimed in claim 14, wherein theelectron beam through hole of the third electrode is smaller than thebeam through hole of the fourth electrode adjacent to the beam throughhole of the third electrode.
 16. An electron gun for a cathode ray tubecomprising: a cathode emitting electron beams; a first electrodecontrolling the electron beams emitted from the cathode; a secondelectrode accelerating the electron beams emitted from the firstelectrode; and third to fifth electrodes sequentially arranged in ascreen direction to act as pre-focus lenses and each having at least oneelectron beam through hole, wherein the electron beam through hole ofthe fourth electrode and the electron beam through holes each includedin the third and fifth electrode and adjacent to the electron beamthrough hole of the fourth electrode are different in size from oneanother, and the size of the electron beam through hole of the thirdelectrode is greater than the size of the electron beam through hole ofthe fifth electrode×0.1 and less than the size of the electron beamthrough hole of the fifth electrode×0.5.